1. Field of the Invention
The present invention relates to a motor control apparatus that controls rotation of a rotor of an electric motor.
2. Description of Related Art
Lately, even in the automobile industry, many mechanical drive systems have been replaced with electrical drive systems, each of which uses an electric motor as a drive source thereof, to satisfy a demand for improving installation space saving of the system, a demand for improving an easiness of assembling of the system, a demand for improving a controllability of the system and/or the like. For example, a range change mechanism of an automatic transmission of a vehicle has been changed to use an electric motor as a drive source thereof (see, for example, Japanese Patent No. 3800529 corresponding to US 2007/0182353A1). In this system, an encoder, which senses a rotational position of the rotor, is provided to the motor. At the time of changing the range of the automatic transmission, the motor is rotated to a target rotational position (a target count value), which corresponds to a target range of the automatic transmission, based on a count value of output pulses of the encoder.
In this type of the motor having the encoder, only the rotational amount (the rotational angle) of the rotor from a start position of the rotor can be sensed based on the count value of the output pulses of the encoder upon the starting. Therefore, the motor cannot be normally driven unless an appropriate measure is taken to obtain a relationship between the encoder count value and the rotational position of the rotor (the energization phase) by sensing an absolute rotational position of the rotor.
In view of the above point, according to Japanese Patent No. 3800529 corresponding to US 2007/0182353A1), an initial position learning process is executed in an initial drive operation right after turning on of an electric power source. In this learning process, the exciting phase(s) of the motor is sequentially changed at a predetermined time schedule through one complete cycle, so that the rotational position of the rotor and the corresponding exciting phase(s) coincide with each other at some timing (at some phase), and the output signals of the encoder is counted upon the rotation of the rotor to learn a relationship among the count value of the encoder, the rotational position of the rotor and the exciting phase(s) at the end of the initial drive operation. Thereafter, in a normal drive operation, the rotational position of the rotor is sensed based on the encoder count value with reference to the result of the learning made at the end of the initial drive operation to determine the corresponding exciting phase(s).
The rotational amount (the rotational angle) of the motor is converted to the rotational amount of an output shaft (the operational amount of a range change mechanism) through a rotation transmission system, such as the speed reducing mechanism. Here, it should be noted that looseness (a play) exists between adjacent components of the rotation transmission system. For example, a play (backlash) exists between adjacent gears of the speed reducing mechanism. Furthermore, in a case where a connecting portion, which has a non-circular cross section (e.g., a polygonal cross section, a D-cut cross section) and is provided to a distal end portion of a rotatable shaft of the speed reducing mechanism, is fitted into an engaging hole of the output shaft, a clearance is required to ease the fitting work for fitting the connecting portion of the rotatable shaft into the engaging hole of the output shaft. As discussed above, since the looseness exists in the rotation transmission system, which converts the rotational amount (the rotational angle) of the motor into the operational amount of the control subject, the rotation of the rotor in the initial drive operation may possibly be executed within a range of the looseness (play), or the motor may be held in the state where the load is applied to the motor upon abutment to one of two delimiting positions (side walls), which define a range of the looseness (play), in the middle of the initial drive operation. When the motor is held in the state where the load is applied to the motor in the initial drive operation, the rotor may possibly not be rotated to the position, which corresponds to the exciting phase(s), even at the end of the initial drive operation, thereby possibly resulting in erroneous learning.